This invention relates to a system for detecting bubbles in a fluid line leading to a patient and, more particularly to an improved means of protecting against the introduction of bubbles into such patient.
One of the common procedures during operations is the monitoring of cardiac output. Although some systems have been proposed to carry out noninvasive cardiac output, by far the most common method is to utilize thermodilution. The method is invasive and utilizes a catheter, such as a Swan Ganz catheter, that is positioned within the heart. A liquid having a known temperature, normally chilled, is introduced through the catheter into the right atrium and a temperature sensor, positioned in the pulmonary artery senses the change in temperature of the blood exiting the heart. By a correlation of time and temperature drop, a measurement of cardiac output can be readily calculated.
In common practice, the injection of the cold bolus of liquid is carried out manually by attending personnel through use of a syringe that has been chilled to the appropriate temperature.
In recent years, there have been attempts to automate the injection of the chilled liquid to improve consistancy and to alleviate the workload of personnel in the operating room. In such systems the bolus of chilled liquid may be automatically introduced by means of a stepper motor controlled syringe or linear actuator. That automated process is referred to as Automated Thermo Dilution (ATD) and is beginning to be used in hospital operating rooms. An example of an ATD system can be seen in U.S. Pat. No. 4,502,488 of Digeronimo.
As a further recent development, the time/temperature relationship used in determining cardiac output can be obtained by an injectless type of system such that the cold liquid injectate does not actually enter the patient's bloodstream. Instead, the cold liquid is contained in a sheath within the catheter, such that heat transfer with respect to the cold liquid and the blood takes place but the liquid is, at all times, isolated from the blood stream. The system is referred to as a Continuous Cardiac Output (CCO) method and is currently undergoing evaluation.
In either system, ATD or CCO, the chilled liquid is pumped by automatic means, thereby eliminating the use of attending personnel to manually operate a syringe. One difficulty with replacing an operator however, is that the injectate systems thereby function without visual observation. Accordingly, a problem can arise by a bubble being present in the chilled injectate and which could move unobserved toward the patient. In the case of ATD, that bubble will, if undetected, ultimately be injected into the patients vascular system where it can cause damage to the patient. The possibility is somewhat more remote in the case of CCO since the system would require a leak somewhere for a bubble to actually enter the patient's bloodstream, however, in the case of medical operations, it is obviously better to provide maximum protection against potential harm to a patient.
There are bubble detectors commercially available that can be used in such injectate systems and many rely upon some photoelectric or ultrasonic means of sensing the bubble. An example of the use of such bubble detectors can be found in U.S. Pat. No. 3,915,155.
A difficulty with such detectors is, however, that the bubble, to be detected, must progress beyond the automatic injection means and therefore time is critical in immediately stopping the further movement of the bubble towards the patient. A rapid means is therefore desired. In addition, conventional bubble detectors, while recognizing and preventing the introduction of small bubbles may not be as effective when dealing with larger bubbles, i.e., 1.0 cc. or more. In such cases, the larger bubbles may contain sufficient energy (pressure) that even if the pumping system is shut down, the bubble will continue to migrate on its own toward the patient. Therefore, conventional bubble detectors may not be sufficiently effective in containing the larger bubbles and in safeguarding the patient.
A further system, shown in EPO Publication 0,328,162, utilized to detect an open line, or air bubble, relies upon an AC signal being present following an infusion pulse with a ringing effect, however, the ringing effect is not prominent in some systems and an AC signal difficult to detect and analyze.